1. Field of the Invention
The present invention relates to a testing system for units such as printed circuit boards, and more particularly to a system which enables a single test apparatus to reliably operate with a plurality of different circuit boards.
2. Description of the Prior Art
For some time computer controlled automatic test equipment has been used to detect faults in printed circuit boards. Generally there is a unique computer test program for each particular circuit board to be tested. It is important to ensure that the proper test program be selected for the particular circuit board, since the electrical stimuli applied by an incorrect test program may cause damage to the circuit board. One method of doing this is by properly labelling the circuit board and providing written instructions to the operator to select a particular program for a certain circuit board. However, this system is subject to human error. Thus, it becomes desirable to have a test system which automatically prevents the wrong test program from being applied to a particular circuit board.
One means of accomplishing this is by providing a hardware interlock system whereby it is not possible to connect a particular circuit board to the test apparatus unless a particular element or elements in the circuit board are able to mechanically interfit with other components associated with the test apparatus. This general sort of mechanical arrangement is known in the Prior Art. For example, U.S. Pat. No. 3,117,461, Hagan et al, discloses a keying device for plug-in circuit boards. There are provided a plurality of socket members mounted to one unit, with these socket members having a predetermined angular configuration. There is mounted to a second unit a plurality of mating keying pins having the same angular orientation as the two socket elements. Thus, the two units can be plugged together only when the sockets and pins have the same angular orientation. By providing various circuit boards with keying pins of different angular orientations, it is possible to have a number of different circuit boards with the same type of electrical connecting elements, but with different keying elements to prevent an incorrect connection of the circuit board.
Other examples of such keying arrangements are shown in the following patents:
U.S. Pat. No. 2,902,665, D'Aimco: U.S. Pat. No. 3,582,867, Thompson et al; U.S. Pat. No. 3,611,272, Fairbairn et al; U.S. Pat. No. 3,818,280, Smith et al; French Pat. No. 1,474,721.
Also, a technical disclosure bulletin published by International Business Machine, Volume 15, No. 21, July 1972 discloses an alignment pin having a particular fixed orientation to permit the joining together of certain units.
Mechanical interfacing devices such as those described above have been used in the field to prevent insertion of printed circuit boards into incorrect sockets in assemblies which contain a plurality of sockets, each of which is otherwise identical. These interfacing devices have also been proposed for applications such as connecting circuit boards to test equipment. Generally, this would involve the use of a unique interface adapter placed between the circuit board and the test equipment, and this adapter would contain a hard wired electrical circuit, thus enabling the test equipment to determine which adapter, and thus which circuit board, has been connected for the particular test program selected. However, the advancement in the state of the art in automatic test equipment has caused a situation wherein a single adapter may be used for many different circuit boards, each of which has a similar connector configuration. Accordingly, it is the object of the present invention to provide a test system able to reliably test a number of different circuit boards requiring different test programs, without requiring specialized apparatus for each particular type of circuit board being tested.